Portable power banks have become essential accessories for travelers, photographers, and remote workers. They promise extra runtime for phones, tablets, and even laptops. Yet the marketing numbers on the box rarely explain how those watt-hours translate into real-world device charges. This calculator fills that gap. By entering the energy capacity of the power bank and the device, along with the conversion efficiency and the device's charging power, you can estimate how many full recharges are possible and how long each charge will take.
The core concept is energy conservation. A power bank stores a certain number of watt-hours. When that energy is transferred to a device, some is lost as heat in the voltage regulation circuitry and cables. The efficiency percentage captures this loss. The usable energy for recharging is the power bank capacity multiplied by the efficiency. Dividing this usable energy by the device's battery capacity gives the number of full recharges. The charging time depends on the power delivered to the device; dividing the device's capacity by the charge power yields the hours required per charge.
Mathematically, the number of charges can be expressed as:
In plain language, multiply the power bank capacity by the efficiency (expressed as a decimal) and divide by the device capacity . The charging time for each cycle, , is , where is the charging power. The calculator handles these conversions automatically.
Consider a worked example. Suppose you have a 100 Wh power bank and want to recharge a 12 Wh smartphone. Assuming 85% efficiency, the usable energy is 85 Wh. Dividing by 12 Wh yields about seven full charges. If the phone charges at 18 W, each charge takes roughly 0.67 hours, or about forty minutes. The calculator will display these numbers and also show a table comparing other power bank capacities.
The table below uses the default device and efficiency inputs and shows how many charges are available from different power bank sizes. This helps illustrate diminishing returns; doubling capacity does not always double usefulness due to weight, cost, and airline restrictions.
| Bank Capacity (Wh) | Full Charges | Total Charging Hours |
|---|
Power bank marketing often lists capacity in milliamp-hours (mAh). To convert mAh at a nominal 3.7 volts to watt-hours, multiply by 3.7 and divide by 1000. For example, a 20,000 mAh bank is roughly 74 Wh. Understanding this conversion is crucial for comparing advertised capacities with actual energy output.
Limitations of this calculator include assuming constant charging power and ignoring device tapering, where charging slows as the battery fills. It also assumes the power bank can deliver the necessary voltage and current for the device. Some phones require specific fast-charging protocols that may reduce efficiency or limit power if not supported. Furthermore, the efficiency value can vary with load; manufacturers rarely publish detailed curves, so the percentage is an estimate.
Despite these caveats, the calculator is a handy planning tool. It can help you decide whether a compact 50 Wh bank suffices for a weekend trip or whether you need a larger model. It also reveals how much time you'll spend tethered to the charger versus enjoying your device.
For related tools, try the Portable Power Station Solar Recharge Time Calculator and the Portable Projector Battery Life Calculator. Both explore portable energy storage in different contexts.
By quantifying realistic expectations, this calculator helps reduce range anxiety for electronics and encourages efficient energy use on the go.
Another nuance is the effect of temperature on lithium-ion chemistry. Capacity ratings are measured at room temperature, yet cold environments can reduce usable energy by as much as 30%. If you plan to use a power bank while camping in winter, your real-world recharge count may be lower than the calculator predicts. Conversely, high temperatures can accelerate aging, so storing the bank in a hot car may permanently reduce its capacity.
The electrical characteristics of cables also matter. Thin or excessively long USB cables introduce resistance that wastes power as heat and slows charging. While these losses are usually small, they can add up over multiple charging cycles. Some enthusiasts choose premium, thicker cables to minimize voltage drop.
This calculator intentionally excludes cost calculations to keep the interface simple, but you can estimate the electricity cost of refilling your power bank by multiplying its capacity by your utility rate. For most small banks, the cost per full recharge is only a few cents, yet understanding it can help frequent travelers budget for energy use.